Methinks he's looking in the wrong places.posted by porpoise at 9:33 PM on January 10, 2009

That Zimmer piece was really disappointing. Most of the article is about how beyond the obvious birth defects, we have little idea about how IQ and genetics are linked. But throughout he keeps on claiming that it is a proven fact that they are linked. His only evidence for this is twin studies, but what twin studies actually measure is highly disputed. Plus it is ridiculous to have a whole article on IQ and heredity without mentioning the Flynn affect.

1. The most common formulae used to estimate heritability are wrong, either for trivial mathematical reasons (such as the upward bias in the difference between monozygotic and dizygotic twins' correlations), or for substantive ones (the covariance of monozygotic twins raised apart neglects shared environments other than the family, such as maternal and community effects).
2. The best estimate I can find puts the narrow heritability of IQ at around 0.34 and the broad heritability at 0.48.
3. Even this estimate neglected heteroskedasticity, gene-environment interactions, gene-environment covariance, the existence of shared environment beyond the family, and the possibility that the samples being used are not representative of the broader population.
4. Now that people are finally beginning to model gene-environment interactions, even in very crude ways, they find it matters a lot. Recall that Turkheimer et al. found a heritability which rose monotonically with socioeconomic status, starting around zero at low status and going up to around 0.8 at high status. Even this is probably an over-estimate, since it neglected maternal effects and other shared non-familial environment, correlations between variance components, etc. Under such circumstances, talking about "the" heritability of IQ is nonsense. Actual geneticists have been saying as much since Dobzhansky at least.
5. Applying the usual heritability estimators to traits which are shaped at least in part by cultural transmission, a.k.a. traditions, is very apt to confuse tradition with genetics. The usual twin studies do not solve this problem. Studies which could don't seem to have been done.
6. Heritability is completely irrelevant to malleability or plasticity; every possible combination of high and low heritability, and high and low malleability, is not only logically possible but also observed.
7. Randomized experiments, natural experiments and the Flynn Effect all show what competent regressions also suggest, namely that IQ is, indeed, responsive to purely environmental interventions."posted by afu at 10:12 PM on January 10, 2009 [6 favorites]

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